The chemical composition of planets and other solar system bodies provides information about their origins and evolution. Small bodies (e.g., asteroids, comets, and the moons of Mars) preserve a record of the early solar system. Indeed, the two largest bodies in the main asteroid belt—Ceres and Vesta—are thought to be surviving members of a population of planetary ‘embryos,’ from which the planets originally grew These two asteroids are chemically distinct and have followed very different evolutionary paths. Vesta formed under hot conditions and consists of magmatic materials that cooled to form a layered interior (i.e., a core, mantle, and crust). Ceres, however, is a water-rich body that probably formed later—or further away from the Sun—than Vesta. Small asteroids may contain a mixture of materials from different parent bodies. Since some of these smaller asteroids are relatively easy to access (e.g., the near-Earth asteroids), they may be potential targets for mining and exploration. Some are potentially hazardous objects. A thorough understanding of their physical and chemical properties, as well as their interior structure, is required for planetary defense.
Nuclear spectroscopy is commonly used to determine the elemental composition of the surface of planets, asteroids, and comets. Gamma rays and neutrons are produced by the interaction of galactic cosmic rays with the surface materials on small airless bodies and planets with thin atmospheres, such as Mars (see FIG. 1). FIG. 1 illustrates gamma-ray production processes in the surface of planetary bodies. Galactic cosmic rays—mainly high-energy protons (p)—smash apart atoms in the top surface of airless bodies. This produces a shower of secondary particles, which include neutrons (n). These neutrons then undergo successive collisions with atoms in the surface and produce gamma rays (y) via inelastic scattering and radiative capture. Gamma rays can also be produced by the decay of natural radioelements, such as potassium (K), thorium (Th), and uranium (U). The spectrum of the gamma rays (see FIG. 2) can be analyzed to determine the concentration of the detected elements. Important rock-forming elements include oxygen (0), magnesium (Mg), silicon (Si) and iron (Fe). In addition, the escaping neutrons convey information about the hydrogen content (H), average atomic mass (A), and the neutron macroscopic capture cross section (Σeff) of the surface materials.
Gamma rays produced via interactions with major rock-forming elements (e.g., iron, silicon, magnesium, and aluminum) provide crucial information for geochemical studies. For planets that have thick atmospheres (e.g., Venus) the flux of cosmogenic gamma rays near the surface is relatively low. All natural solar system materials, however, contain long-lived radioelements (e.g., potassium, thorium, and uranium) that are an additional source of characteristic gamma rays. The potassium/thorium ratio of surface materials is a diagnostic measure of a body's bulk composition and can provide information about where and when the planet or asteroid formed. In planetary nuclear spectroscopy experiments, the emitted gamma rays and neutrons are measured by a spectrometer that is deployed either on the surface (i.e., on a rover or lander) or in a close-proximity orbit within about one body radius of its surface. Long accumulation times (months) are normally required to achieve measurements with the precision required to map elemental surface compositions from orbit. A number of missions to the Moon, Mars, Mercury, and the asteroid 4 Vesta have been used to successfully obtain elemental mapping data (see FIG. 2). In situ surface measurements have also been made for Venus, the asteroid Eros, and Mars. FIG. 2 illustrates a comparison of the average lunar gamma-ray spectrum, as acquired during the Lunar Prospector (bottom graph) and Kaguya (upper graph) missions. Bismuth germanate (BG0) was used for the detector in the Lunar Prospector instrument, whereas high-purity germanium (HPGe) was used for the Kaguya spectrometer. Additional information about the data can be found in T. H. Prettyman, J. J. Hagerty, R. C. Elphic, W. C. Feldman, D. J. Lawrence, G. W. McKinney, D. T. Vaniman, Elemental composition of the lunar surface: analysis of gamma ray spectroscopy data from Lunar Prospector, J. Geophys. Res. 111, p. E12007, 2006.doi:10.1029/2005JE002656, and N. Yamashita, O. Gasnault, O. Forni, C. d'Uston, R. C. Reedy, Y. Karouji, S. Kobayashi, et al., The global distribution of calcium on the Moon: implications for high-Ca pyroxene in the eastern mare region, Earth Planet. Sci. Lett. 353-354, p. 93-98, 2012, both of which are fully incorporated by reference and made a part hereof. Prominent gamma-ray peaks in the spectra indicate the presence of various elements in the target. These include aluminum (Al), calcium (Ca), and titanium (Ti). The two insets show global maps of iron oxide (right) and calcium oxide (left) created from Lunar Prospector and Kaguya data, respectively. These maps show concentration ranges of 8-20 wt % for calcium oxide and 2-25 wt % for iron oxide. The peak labeled 12C* is made by neutron reactions with oxygen or carbon.
Planetary nuclear spectroscopy applications require that instruments have high gamma-ray detection efficiencies, as well as energy resolutions that are sufficient to distinguish contributions from different elements. The requirement for high efficiency emphasizes the need for large-volume detectors, scintillators, and semiconductors that have relatively high atomic numbers and densities. Cosmogenic and radiogenic gamma-ray emissions from planetary surfaces are relatively weak. Therefore, detectors must have low self-activity levels. For example, gamma rays produced from the decay of radio-lanthanum in the scintillator lanthanum bromide (LaBr3) can obscure gamma-ray emissions from the Moon. In addition, the harsh radiation conditions of space mean materials that are insensitive to radiation damage, or for which the damage can be mitigated (e.g., via annealing), are preferable. Spaceflight applications also require simple, low-power, compact, and rugged electronics.
Flight-heritage gamma-ray spectrometers have included various low-resolution scintillators for detection, e.g., bismuth germanate that was flown on the Lunar Prospector and Dawn missions. High-purity germanium (HPGe)—a semiconductor—was used in the gamma-ray spectrometers that were flown on the Kaguya, Mars Odyssey, and MESSENGER missions. The scintillator materials have the advantage of being relatively inexpensive. They can also be deployed at ambient temperatures, although this is at the cost of reduced sensitivity for some elements. HPGe, however, can be used to achieve very high spectral resolutions and therefore provide high levels of elemental sensitivity. This benefit, however, does come with the expense of added complexity, cost, and bulk, which are associated with the necessary cryogenic cooling. Although HPGe is seen as the ‘gold standard’ for gamma-ray spectroscopy detectors, we highlight that high resolution does not always guarantee optimal instrument performance. Additional performance-limiting factors include the mission concept of operations, design measures to mitigate backgrounds and the deployment of the instrument on the spacecraft.
For additional background material, please refer to “Ultra-bright scintillators for planetary gamma-ray spectroscopy,” Thomas Prettyman, Arnold Burger, Naoyuki Yamashita, James Lambert, Keivan Stassun and Carol Raymond, 23 Oct. 2015, SPIE Newsroom. DOI: 10.1117/2.1201510.006162, which is incorporated by reference.
Therefore, what is desired is a gamma-ray spectroscope that overcomes disadvantages in the present state of the art, some of which are described above.